Packaging is a major area for use and consumption of impact polystyrene resins. Articles of impact polystyrene for packaging are often formed by extrusion of the polystyrene into sheet followed by multicavity thermoforming of the sheet into cups or tubs with deep draw thermoforming. For such service the polystyrene must be capable of being extruded into sheet with good impact properties in the machine direction and in the direction transverse of the sheet, and should exhibit nearly balanced elongation and impact values in both sheet directions. This sheet also must be capable of being deep drawn by having sufficient and uniform melt strength to permit the formation of containers with uniform wall thickness changes. The extruded sheet must also be free of gels and its rubber particle size distribution should be uniform so as to form a sheet of uniform appearance.
Desirably the impact polystyrene resin should exhibit a high rubber efficiency, i.e. provide a high level of impact resistance for a given increment of rubber present. It is advantageous from the manufacturer's standpoint that the impact properties should increase continuously with rubber level to as high a rubber level as possible. Further, for use in sheet forming and a variety of packaging applications, impact polystyrene having a high rubber level should possess good impact strength and elongation when blended with crystal polystyrene to lower rubber levels.
In the past it has been difficult to obtain an increase in impact strength when rubber levels exceeded six percent. Polystyrenes with greater than 6% rubber seldom could be blended down with crystal polystyrene to a 6% rubber level to obtain a polymer with impact equivalent to that made directly at 6% rubber by grafting. This "blend back" capability is important since crystal polystyrene has a shorter manufacturing cycle time than impact polystyrene. Hence a given reactor volume can produce more resin in a given time by using blend back techniques to employ the reactor volume to produce more crystal polystyrene and less impact polystyrene. A useful high rubber level impact polystyrene must not only possess a high rubber level but this rubber level must be effective in increasing impact so that when it is diluted with crystal polystyrene useful high impact polystyrenes are obtained.
The rubber particle itself also must be able to resist deformation in extrusion shearing to avoid appearance changes in machine and transverse directions. High cis polybutadiene is commercially available with a molecular structure and branching capable of giving lower rubber solution viscosity at a given rubber level than polybutadiene polymerized with a lithium catalyst and having about 10% vinyl, and 35% trans polybutadiene. However, while the latter compound has been polymerized into useful high impact polystyrenes with deep draw characteristics, i.e. high melt strength, the high cis rubbers which form lower rubber solution viscosities at a given weight of rubber have had limited use in polymerization with styrene to make polystyrene suitable for thermoformed packaging since their deep draw characteristics suffer due to limited melt strength. The melt strength of an impact polystyrene appears to be related to the molecular weight of the rubber, the cross-linking of the rubber particle and the grafting of the rubber particle with high molecular weight polystyrene.
In manufacturing high impact polystyrene, mass polymerization of the rubber polystyrene solution is employed even with solutions which are subsequently suspension polymerized. However, in rubber-styrene solution systems which are subsequently suspension polymerized a high viscosity solution is more difficult to stir and break down into high impact polystyrene bead sizes of 0.1 to 1 mm which can be handled in manufacturing equipment such as centrifuges and dryers than a low viscosity solution. Thus conventional stirring equipment forces a practical limitation on rubber content, molecular weight of the polystyrene, and conversion level of styrene into polystyrene at suspension because of viscosity constraints. When rubber is dissolved in styrene without a chain regulator present or only a limited amount present, there is a considerable risk of the formation of gels in the rubber solution, particularly if a high molecular weight rubber is employed. The further the conversion proceeds without a chain regulator such as t-dodecyl mercaptan present, the greater the risk of gels and very high viscosity rubber solutions. As the polystyrene rubber solution is heated thermal polymerization proceeds but only a limited amount of grafting takes place. Addition of catalyst can be made to promote grafting but catalyst addition to a rubber solution in polystyrene increases the polymerization rate of the rubber-styrene solution and the rate at which the viscosity increases. Catalyst addition also reduces the rubber particle size by the increase of styrene grafting to the rubber. If too much catalyst is added the rubber particle size can become too small for optimum impact properties and the rate of polymerization in large commercial reactors of 5000 gal. to 15,000 gal. size may become too rapid for control of temperature with reactor cooling jackets. Even worse, the rate of viscosity increase may be so great that it is difficult to suspend the reaction mass at the time that suspension may be effected with the stirring systems employed. The thermal polymerization method employed for the mass polymerization has the advantage that it proceeds at a controlled rate of conversion dependent on temperature and with polymer molecular weight formation which is dependent on temperature and secondarily on conversion. With thermal polymerization the phase inversion and the viscosity changes occurring can be observed. Also, the rate of reaching a conversion point which is beyond phase inversion and which still possesses a manageable viscosity for suspension is controllable. It would seem possible to avoid these viscosity limitations by early suspension; however, too early suspension can lead to excessively high moisture in the beads obtained from suspension and incomplete drying in plant dryers. Additionally, the phase inversion step must be completed to assure rubber particle size control to a uniform and reproducible size distribution.
The addition of catalyst in the mass stage can cause poor temperature control, form rubber particles which are too small in size (too many rubber particles below 1 micron in diameter) and make difficult the detection and selection of the proper suspension time.
At some point, it is essential to add catalyst to effect rapid polymerization, cross-linking, and grafting of styrene and rubber. Otherwise, grafting and cross-linking is limited and properties of the impact polystyrene suffer. Also, catalyst is required to shorten the cycle time for conversion of the remaining styrene to polymer. Catalyst addition is normally made after suspension in order to handle the high viscosity polymer as a bead and to control temperature of the beads as a slurry in water.